This invention relates to coil springs and to thermally actuated cut-off links (also referred to commonly as thermal fuses or cut-offs) and normally open switches in which such springs are incorporated. Such cut-off links and switches respond to the ambient temperature surrounding the same by opening or closing an electric circuit when the ambient temperature reaches a control value. Normally, closed versions of such thermally actuated cut-off links, for example, are frequently physically incorporated into the windings of electric motors and in other devices requiring thermal protection and electrically connected in series with such devices so that the cut-off links will de-energize the devices involved when the ambient temperature exceeds a given safe value.
Ambient thermally actuated cut-off links commonly comprise a metal casing and a sandwich of elements including a fusible pellet, contact-making elements, and springs which impose forces on the elements including the pellet which cause a desired rapid opening of the contacts involved when the ambient temperature involved reaches an undesired level which melts the fusible pellet. It is important that slight creeping of the pellet does not materially change the opening characteristics of the cut-off link. Usually the springs referred to comprise relatively weak and strong coil springs, the stronger of which keeps the weaker one fully compressed and takes up the slack in the sandwich of elements caused by tolerance variations and creeping of the pellet. It is especially desirable that the latter spring be compact as well as being capable of following any pellet creeping.
Using conventional coil springs for this purpose was found unsatisfactory in a recently developed cut-off link construction where one and preferably at least two outwardly inclining deformable contact-forming arms were preferably formed at the inner end of a power lead and forced against a backing member to expand the arms into good contact-making engagement with the conductive inner surface of the casing. The backing member forms one of the elements of a sandwich of elements extending between the casing ends. This sandwich of elements comprises a fusible pellet, a pressure distributing disc, a first partially compressed spring for taking up any play in the sandwich of elements, said backing member, arm-deforming means positioned contiguous to and adapted to collapse said arms when released to do so, and a second fully compressed spring held fully compressed by the first spring and applying a force acting on said deforming means to force it into an arm-collapsing position when the pellet melts.
In the initial design of the spring, the partially compressed spring was a conical spring whose wide end pressed against the peripheral portions of the pressure-distributing disc. The specifications of fuse manufacturers for cut-off links places a maximum limit on the overall size of the cut-off links. It was found that using conventional conical or cylindrical coil springs that, to obtain the necessary strength and expandability of such a spring to keep the other coil spring fully compressed, required a spring of such a length that it was not possible to make a cut-off link with the desired small length.
In accordance with the present invention, a unique coil spring was invented which enables this spring to be of exceedingly small size in its collapsed condition, while having the capability of following any creeping of the fusible pellet and retaining sufficient force to keep the other coil spring fully compressed. This hour glass shaped coil spring is preferably made of music wire. The coil spring has outermost spiral turns which are the coils of maximum diameter, and off-centered turns of lesser diameter between the same. The various turns of the coil are off-centered in a manner so that when the coil is compressed, the contiguous portions of the turns will overlap and nestle together, so that the longitudinal dimensions of the compressed spring are reduced from that of a conventional coil spring.